1. Field of the Invention
The present invention relates in general to a data output buffer used in a semiconductor memory device, and more particularly, to a data output buffer which is capable of minimizing an impulse noise component in output data on a data output line by controlling an amount of current flowing through a pull-down transistor according to a logic state of an input data signal from a memory cell when the output data on the data output line is low in logic.
2. Description of the Prior Art
A data output buffer of the present invention controlling an amount of current flowing through a pull-down transistor in a multi-step manner can be applied to all highly integrated circuits.
Referring to FIG. 1, there is shown an example of conventional data output buffers. In the case where output data on a data output line N3 is low in logic, a difference between a voltage on a node N2 and a ground voltage from a ground voltage source Vss becomes large, thereby causing a pull-down transistor Q2 to be turned on. As the pull-down transistor Q2 is turned on, a voltage on the data output line N3 is transferred to the ground voltage source Vss. At this time, an amount of current flowing through the pull-down transistor Q2 to the ground voltage source was instantaneously varies abruptly, resulting in the generation of a very high impulse noise component in the low logic output data on the data output line N3.
On the other hand, in the case where a control signal OE is high in logic and an input data signal DOb on a data input line is low in logic, a pull-up transistor Q1 is turned on, thereby causing the data output line N3 to maintain a high voltage corresponding to a supply voltage. When the input data signal DOb on the data input line goes from low to high in logic, the current flowing through the pull-down transistor Q2 is abruptly increased in amount because of the high voltage on the data output line 3. Namely, the noise component included in the output data on the data output line N3 varies more abruptly as the supply voltage is increased.